In many optical systems, the optical path between two lenses or the optical path between an object and an image is folded to reduce a dimension of a mechanical enclosure. For example, binoculars commonly use a pair of prisms to fold the optical path between two lenses to reduce the overall device length (and also to provide an upright image). Some telescopes, copiers, and optical image scanners also reduce a device dimension by incorporating mirrors, prisms, and other optical components to fold an optical path back and forth within the device.
FIG. 1A illustrates an example folded optical path in a prior art optical image scanner. A lens 100 focuses a single line from a document 102 onto an image at a sensor array 104. Light from the document is reflected from a first mirror 106, a second mirror 108 and third mirror 110 before entering the lens 100. If length is defined as a dimension parallel to segment 118, then the system shown reduces the length of the system. However, the reduction in length comes at the expense of increasing the orthogonal dimension labeled as "D" (120) in FIG. 1. Mirror 110 must be non-perpendicular to segment 118 and mirror 108 must be non-perpendicular to segment 114. Stated alternatively, path segments 114, 116, and 118 cannot be coplanar. Similarly, binoculars often decrease the length of the binoculars as measured along the sight path from the eyes of the human observer, at the expense of making at least part of the binoculars substantially wider than the spacing of human eyes.
FIG. 1B illustrates a top view of the optical system of FIG. 1A. FIG. 1B illustrates that light rays forming an image at sensor 104 converge through the center of lens 100.
In the case of scanners and copiers, there is an ongoing need to make the products smaller. In particular, there is a need to make the optical systems smaller, lighter and lower cost. Typically, the optics shown in FIG. 1 are contained within a moveable carriage. Reducing the size of the carriage enables a reduced product height, and reduces the weight of the carriage which in turn enables use of smaller, lower cost motors for moving the carriage and lighter supporting structures to hold the carriage. There is a need for a folded optical system that reduces the length of the system without increasing a dimension orthogonal to the length.